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Post-Retirement Liabilities - Additional Information (Detail) - BRL (R$)
R$ in Millions
 1 Months Ended 12 Months Ended
Feb. 28, 2018
May 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of defined benefit plans [line items]          
Obligation for past actuarial deficits relating to pension fund     R$ 721 R$ 787  
Obligation for past actuarial deficits amortization description     Being amortized by June 2024, through monthly installments calculated by the system of constant installments (known as the 'Price' table), and adjusted by the IPCA (Expanded National Customer Price) inflation index (published by the Brazilian Geography and Statistics Institute - IBGE) plus 6% per year.    
Obligation rate for actuarial assumption     6.00%    
Costs of post-employment obligations     R$ 65 103 R$ 129
Financial costs and monetary updating expence     65 103 129
Life insurance, a recovery of expense     619   74
Post Employment Benefit Obligations [Member]          
Disclosure of defined benefit plans [line items]          
Costs of post-employment obligations     391 R$ 345 R$ 230
Pension Funds [member]          
Disclosure of defined benefit plans [line items]          
Expected employer contributions     166    
Defined contribution plan [member]          
Disclosure of defined benefit plans [line items]          
Expected employer contributions     R$ 90    
Entering into significant commitments or contingent liabilities [member]          
Disclosure of defined benefit plans [line items]          
Remuneratory interest 6.00%        
deficit found in Plan R$ 99        
Monthly installments 167 monthly installments        
Forluz pension [member]          
Disclosure of defined benefit plans [line items]          
Retirement benefits plan   R$ 283      
Remuneratory interest   6.00%